Hello, everyone!

Some C projects use a construct called a "flexible array". One example is name_to_handle_at() and open_by_handle_at() API on Linux, which uses a struct file_handle defined as:

struct file_handle
{
    unsigned int handle_bytes;
    int handle_type;
    unsigned char f_handle[];
}

The last member is a char array of arbitrary size (determined by handle_bytes). In Rust, there is a feature that allows making a custom dynamically-sized type, by making a DST the last element in the struct. It can be used, for example, as Arc<RwLock<dyn SomeTrait>>, to allow wrappers over trait objects, or by core::str::CStr, to allow referencing a byte-array checked for 0-termination with &CStr (which is done using pointer casts), in the same way that a byte-array checked for UTF-8 validity can be referenced with &str. Rustonomicon mentions that such type can be created using generic unsizing coercion (such as array-to-slice). However, I wonder, if the size is not known at compile-time, as far as I understand, the only option is allocating memory manually by calling unsafe alloc() with necessary Layout. I wonder, what is the correct way to then create a boxed custom-DST object and can such objects (or, more precisely, the "address" part of the fat pointer) be passed to C API?

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